Untreated CD20-positive, B-cell non-Hodgkin's lymphoma in the patients with seronegativity for HBsAg and seropositivity for HBc-Ab and/or HBs-Ab, which is planned to be treated with 6-8 courses of systemic chemotherapy containing rituximab plus steroid such as R-CHOP, R-CVP, R-THP-COP and R-C-MOPP